The stream cuts through late Cretaceous-Tertiary sedimentary and igneous rocks and Quaternary floodplain conglomerate and colluvium. In the area of Kaikorai Valley College the bedrock is the Miocene (c.17 Ma millions years ago) Caversham Sandstone.
'Baked' rock layers
A special feature of Kaikorai Valley is the boundary between the Caversham Sandstone (a sedimentary rock) and the overlying Dunedin Volcanic Group (basalt igneous rocks).
An old brown-black soil layer called a palaeosol occurs on top of the Caversham Sandstone. Since the Caversham Sandstone has fossils in it that tell us it was deposited under the sea, and soils develop on land, we know the sea must have retreated from this area after deposition of the Caversham Sandstone - an unconformity between the two is the inferred.
In many places the palaeosol and the Caversham Sandstone have been 'baked' by the hot lava that was lain down on top (Figure 1). We can see this by looking at carbonised plant material and the structure of the soil layer. This lava then cooled to become the rock of the Dunedin Volcanic Group. An unconformity occurs between the sandstone and the volcanics. In some places superb columnar jointing can be seen in the rocks of the Dunedin Volcanic Group (Figure 2).
Figure 1. The boundary between the Caversham Sandstone and the Dunedin Volcanic Group in Kaikorai Valley; palaeosol which is about 30 cm thick occurs between these very different rock types
The difference in age between the Caversham Sandstone,
which at this locality, is about 17 million years old and the Dunedin
Volcanic Group rocks which are about 14 million years old, is a lot. This
means there is about 3 million years that is represented by the palaeosol
layer and the unconformities!
Figure 2. The columnar jointing in basalt igneous rocks of the Dunedin Volcanic Group in Kaikorai Valley
Elsewhere, 2km north-westward in Fraser's Gully, fine-grained silica-rich sedimentary (diatomite) layers occur which have galaxid fish and leaf fossils in them (Figure 3, Campbell 1985).
It is likely these formed at a similar time to the
palaeosol layer. This helps us understand that a land surface (like we
live on today) with soils, trees and shrubs and a lake existed on top
of the Caversham Sandstone between 17-14 million years ago; before and
probably during the volcanic events that led to creation of the Dunedin
Figure 3. Galaxid fish fossil from the Kaikorai Leaf Beds c. 14-16 million years old
Some areas of land are unstable and have in the past or still are moving! This happens in the lower Kaikorai Valley in particular (McKellar 1990).
The Abbotsford Landslide that happened in 1979 is just one example of many (Coombs & Norris 1981, Figure 4). Sometimes houses and roads are affected resulting in substantial damage (Figure 5).
The landslides have occurred for many different reasons,
but all are in sedimentary rocks and many of these have a high amount
of swelling clay called smectite in them. These clays become plastic when
wet and have low strength.
Figure 4. Map showing outline of the East Abbotsford Landslide 1979 in lower middle Kaikorai Valley, shown here by the red X.
Figure 5. The devastating chasm that opened on the hillside at Abbotsford in 1979. These houses were formerly on Edward Street.
The lower Kaikorai Valley has been an important source of natural resources in the past. Gold and scheelite were mined (pre 1880) in the Haast Schists at the south end of the Chain Hills, coal in the Fairfield area (pre 1960), quartz gravel and sands also in the Fairfield area (current), clay at Abbotsford, and marl (fine-grained calcium carbonate rich claystone pre 1990) for cement manufacture at Burnside.
In the early days of settlement in Kaikorai Valley settlers reported that the area was rich in bird and plant life. A remnant forest was set aside in Frasers Gully in the upper part of the Kaikorai Catchment. This is now a public reserve. In the reserve you can still see large totara and all sorts of other native trees.
Figure 6. Special features of Kaikorai Valley: Beech fossils from the Kaikorai Leaf Beds - about 14-16 million years old
Campbell, J. D. (1985) Casuarine, Fagaceae and other plant megafossils from Kaikorai Leaf Beds (Miocene), Kaikorai Valley, Dunedin, New Zealand. New Zealand Journal of Botany 23: 311-320.
Coombs, D. S. and Norris, R. J. (1981) The East Abbotsford, Dunedin, New Zealand Landslide of August 8th 1979. An interim report. Bull. Liaison Labo. P. et ch., Special X, Janv.
McKellar, I. C. (1990) Southwest Dunedin Urban Area. New Zealand Miscellaneous Series Map 22: 1:25000. New Zealand Geological Survey DSIR, Lower Hutt, New Zealand.